Hypophosphatemic rickets (HYP) is the most common inherited form of rickets in the United States and the most common inherited form of hypophosphatemia. Identification and characterization of specific genetic defects in PEX, the recently identified gene for x-linked (HYP), will be the first goal of this study. The second goal is to identify genes other than PEX that may lead to the HYP phenotype in patients with no mutations in PEX. Since this is a study looking at genetic analysis of a rare disease, there is no final number of subjects projected.